There are various recording media for recording and storing digital audio, images, videos, and document files or data files created on a computer. One of such media is an optical disk. In particular, DVDs (Digital Versatile Disks) have a higher density and a greater capacity than conventional CDs (Compact Discs), and are becoming popular in the field of recording devices as the medium to succeed currently dominant VTRs (Video Tape Recorders). Furthermore, in recent years, research on next-generation optical disks which use a blue-violet semiconductor laser and feature further improved recording density is being carried out at various locations, and its early appearance and penetration are anticipated. Moreover, recording/reproducing data with such optical disks require an optical pickup device.
Generally, an optical pickup that records or reproduces information on an optical disk includes a light source, an objective lens that causes a beam outputted from the light source to be collected on the optical disk, and a detector that detects a beam reflected off of the optical disk.
With a semiconductor laser as the light source, since a beam is emitted from an edge face of a thin active layer, the cross-sectional shape of a beam or, in other words, the shape as seen from an optical axis direction has an elliptical shape having a minor-to-major axis ratio of approximately 1:3. When recording information onto an optical disk, an elliptical beam is desirably shaped into a circular shape from the perspective of improving light use efficiency.
An anomorphic prism or an anomorphic lens is generally used as such beam shaping elements. However, since an anomorphic prism must be used in a parallel light beam obtained by collimating diverging light outputted from a semiconductor laser, downsizing of optical pickups is inhibited. In consideration thereof, by using an anomorphic lens capable of performing beam shaping on a divergent light beam, an increased light use efficiency and a smaller-sized optical pickup can be realized.
On the other hand, other elements that actually constitute an optical pickup include a reflective optical element and an objective lens. However, these elements may generally have astigmatism as an initial characteristic. While a beam shaping element featuring an anomorphic lens characteristically generates astigmatism due to a displacement from a luminous point with respect to a design value, an optical pickup conversely takes advantage of this characteristic and is capable of correcting astigmatism existing in a reflective optical element or an objective lens. Such a configuration enables the quality of spots outputted from the objective lens of an optical pickup and formed on the optical disk to be improved.
FIG. 24 shows a diagrammatic explanatory drawing depicting a configuration of a light source portion of a conventional example of a beam shaping element retaining structure (for example, refer to Japanese Patent Laid-Open No. 2003-178480). FIGS. 24(a) and 24(b) respectively represent an xz cross-sectional view and a yz cross-sectional view.
In the present specification, a direction parallel to the optical axis as seen from a luminous point of the light source shall be referred to as a +z direction, a direction perpendicular to the optical axis and parallel to an optical recording medium as seen from the luminous point shall be referred to as a ±x direction, and a direction perpendicular to both the optical axis and the optical recording medium as seen from the luminous point shall be referred to as a ±y direction. Furthermore, it is assumed that the x axis, the y axis and the z axis are perpendicular to each other.
The light source portion disclosed in Japanese Patent Laid-Open No. 2003-178480
includes a semiconductor laser P that emits an elliptical divergent beam; a beam shaping element L that converts an elliptical divergent beam emitted from the semiconductor laser P into an approximately circular divergent beam; and a retaining member H that integrally retains the semiconductor laser P and the beam shaping element L.
The beam shaping element L is constituted by a cylinder face whose first face (S1) and second face (S2) are both curved only in the x direction, wherein both cylinder faces (S1, S2) are curved so as to be a concave face with respect to the semiconductor laser P only on cross sections having smaller capture angles from the semiconductor laser P (i.e., xz cross sections).
The light source portion of the conventional example described above is designed so as to be able to provide a light source device capable of outputting an light beam that diverges while retaining an approximately circular shape and suppressing wavefront aberrations due to changes in environmental temperature by equalizing distances s and t shown in FIGS. 24(a) and 24(b) and by equalizing linear expansion coefficients of the beam shaping element L and the retaining member H.
Moreover, stable configurations of a beam shaping element include the example described in Japanese Patent Laid-Open No. 2004-247032. This configuration is provided with a shape in which both an incident plane and an output plane are cylindrical planes, and shapes an incident beam having an elliptically shaped cross section into an approximately circular shape by expanding a minor axis direction thereof while retaining its major axis size.